This invention is directed to an improved process for the catalytic conversion of a C.sub.9 + aromatic feedstock, optionally containing toluene, in the presence of a certain class of zeolite catalysts under transalkylation/disproportionation reaction conditions to provide a product containing significant amounts of benzene and xylene(s). When compared with other disproportionation/transalkylation processes, the present process provides significant improvement in product xylene/benzene mole ratio.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (U.S. Pat. No. Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
U.S. Pat. No. 4,380,685 discloses the para-selective alkylation, transalkylation or disproportionation of a substituted aromatic compound to provide a mixture of dialkylbenzene compounds employing as catalyst a zeolite characterized by a Constraint Index of 1 to 12 and a silica/alumina mole ratio of at least 12/1, the catalyst having incorporated thereon various metals and phosphorus. Other patents covering alkylation and transalkylation processes include U.S. Pat. Nos. 4,127,616; 4,361,713, 4,365,104; 4,367,359; 4,370,508; and, 4,384,155. Toluene is converted to para-xylene as disclosed in U.S. Pat. Nos. 3,965,207; 3,965,208; 3,965,209; 4,001,346; 3,002,698; 4,067,920; 4,100,215; and, 4,152,364, to name a few. Alkylation with olefins is disclosed, for example, in U.S. Pat. Nos. 3,962,364 and 4,016,218. Toluene shown to be is disproportionated in, for example, U.S. Pat. Nos. 4,052,476; 4,007,231; 4,011,276; 4,016,219; and, 4,029,716. Isomerization of xylenes is disclosed in, for example, U.S. Pat. Nos. 4,101,595; 4,158,676; 4,159,282; 4,351,979; 4,101,597; 4,159,283; 4,152,363; 4,163,028; 4,188,282; and, 4,224,141.
U.S. Pat. No. 3,551,509 and U.S. Pat. No. Re. 27,639 disclose transalkylation between trimethylbenzenes and toluene to yield xylenes and benzene in the presence of a crystalline aluminosilicate catalyst having large pore openings of 8 to 15 Angstrom units and preferably containing Group VIII metals, hydrogen and rare earth cations.
U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879; and, 3,607,961 describe the vapor-phase disproportionation of toluene over various catalysts. U.S. Pat. No. 4,117,026 discloses disproportionation of toluene over a catalyst comprising a zeolite having a silica/alumina mole ratio of at least 12, a Constrain Index of 1 to 12 and a specified sorption capacity for xylenes.
U.S. Pat. No. Re. 31,781 (of original U.S. Pat. No. 4,100,214) discloses the use of from 3 to 30 combined weight percent of toluene and C.sub.9 + recycle material as diluents with a monocyclic alkyl aromatic hydrocarbon feed selected from the xylenes, mesitylene, durene, hemimellitene, pseudocumene, prehnitene, isodurene and 1,3,5-triethylbenzene for the vapor-phase isomerization of said feed employing as catalyst, a zeolite having a Constrain Index of 1 to 12, e.g., ZSM-5, ZSM-11, ZSM-12, ZSM-35 and ZSM-38.